Replace two uses of build_array_helper with a new recurse_elem variant

For the item index generation and the member population for the C++
structs, a level-order variant of `recurse_elem` is sufficient - as also
indicate by the unused item index parameters.

sixtyfps_compiler/generator/cpp.rs

@@ -1170,7 +1170,7 @@ fn generate_component(
 
     let mut repeater_count = 0;
 
-    super::build_array_helper(component, |item_rc, _, _| {
+    crate::object_tree::recurse_elem_level_order(&component.root_element, &mut |item_rc| {
         let item = item_rc.borrow();
         if item.base_type == Type::Void {
             assert!(component.is_global());

sixtyfps_compiler/object_tree.rs

@@ -1419,6 +1419,24 @@ pub fn recurse_elem<State>(
     }
 }
 
+/// Call the visitor for each children of the element recursively, starting with the element itself.
+/// The traversal happens in level-order, meaning each level of the element tree is visisted before
+/// going to the next depth.
+///
+/// The state returned by the visitor is passed to the children
+pub fn recurse_elem_level_order(elem: &ElementRc, vis: &mut impl FnMut(&ElementRc)) {
+    vis(elem);
+    visit_children(elem, vis);
+    fn visit_children(elem: &ElementRc, vis: &mut impl FnMut(&ElementRc)) {
+        for child in &elem.borrow().children {
+            vis(child);
+        }
+        for child in &elem.borrow().children {
+            visit_children(child, vis);
+        }
+    }
+}
+
 /// Same as [`recurse_elem`] but include the elements form sub_components
 pub fn recurse_elem_including_sub_components<State>(
     component: &Component,

sixtyfps_compiler/passes/generate_item_indices.rs

@@ -10,7 +10,7 @@ LICENSE END */
 //! Assign the Element::item_index on each elements
 pub fn generate_item_indices(component: &std::rc::Rc<crate::object_tree::Component>) {
     let mut current_item_index: usize = 0;
-    crate::generator::build_array_helper(component, move |item_rc, _, _| {
+    crate::object_tree::recurse_elem_level_order(&component.root_element, &mut |item_rc| {
         let item = item_rc.borrow();
         if item.base_type == crate::langtype::Type::Void {
         } else {